The present invention relates to an FM signal demodulating circuit.
A prior art pulse count demodulating circuit, which is an example of an FM signal demodulating circuit, is shown in FIG. 1. In this circuit, an FM signal is fed to a limiter 1, which has its output terminal connected to the input of a monostable multivibrator 2. The multivibrator 2 thus generates pulses of a predetermined width upon the occurrence of falling edges of the output pulses from the limiter 1. A drop-out compensator 4 is connected through a low-pass filter (LPF) 3 to the output terminal of the monostable multivibrator 2. The output signal of the drop-out compensator 4 is representative of a demodulated audio signal of the FM signal.
In the FM signal demodulating circuit thus arranged, the FM signal fed to the limiter 1 is subjected therein to waveform shaping. Specifically, voltages above a first predetermined value and below a second predetermined value lower than the first predetermined value are clamped to thereby provide square pulses. The monostable multivibrator 2 generates reference pulses of a predetermined width in response to the falling edges of the square pulses produced by the limiter 1, and the reference pulses are integrated by the LPF 3 and fed to the drop-out compensator 4. The drop-out compensator 4 outputs the integrated signal of the LPF 3 while holding the same. In this manner, when drop-out occurs, the integrated signal immediately before the occurrence of the drop-out is outputted as the demodulated audio signal from the drop-out compensator 4.
In such a prior art FM singal demodulating circuit, since the reference pulses are outputted upon being integrated by the LPF 3, even in the case where the drop-out period of the FM signal is short, a time longer than the drop-out period is required before the integrated output signal of the LPF 3 returns to a proper level. As a result, the drop-out compensator 4 must provide its output over a period of time longer than that actually needed for drop-out compensation. If, however, the period for continuously outputting the integrated signal is prolonged, there arises a problem that the reproduced demodulated audio signal has an unnatural sound. Moreover, if the time constant of the LPF 3 is made small, high-frequency noise is allowed to pass therethrough, thereby inviting improper sampling such that an accurately demodulated signal cannot be obtained.